Tissue reaction after implantation of a foreign material or device in the body follows a general pattern, independent of the material used. The initial inflammatory response to an implant consists of acute vasculature changes induced by surgical trauma. Neutrophils migrate to the wound site after this trauma. Migration of monocytes follows. These monocytes then differentiate into macrophages. The macrophage can be viewed as a control cell in the inflammatory response, playing a central role in the interaction of inflammatory mediators.
Titanium is a biomaterial that exhibits good biocompatibility and minimal inflammatory response following implantation. Early studies observed that titanium bone implants in animals were well accepted when compared to other metals. Other studies have shown that titanium evokes less tissue reaction in rabbit muscle as compared to other metals. Clinical trials performed in 1965 indicated a 90% success rate for titanium dental implants (Albrektsson, Branemark et al. 1983). Titanium is used extensively in restorative surgery, particularly as a bone-anchoring and joint-replacement material. Further, there is experimental evidence that titanium as an implant material is less susceptible to infections than stainless steel (Johansson et al., 1999).
Given the normal initial tissue response to titanium, it would be expected that titanium implants would produce a typical inflammatory response. Macrophages near titanium implants, however, do not appear activated and leukocytes associated with titanium implants are less responsive. Although titanium is widely used as an implant material, the mechanisms of its superior biocompatibility are currently unknown.
One explanation is that the surface of titanium implants down regulate the inflammatory response (whose initiation is inevitable) by preventing its prolongation, thereby reducing the overall tissue response and allowing healing to proceed. Titanium readily forms a stable surface layer of oxide upon exposure to air, predominantly consisting of titanium dioxide, TiO2. This oxide layer of titanium is the surface encountered by inflammatory cells after insertion of the implant. It has been proposed that the oxide layer plays a fundamental role in tissue response (Albrektsson, Branemark et al. 1983) and the oxide layer has been regarded as important because of its corrosion resistance properties.
Polymer and glass surfaces have been coated with titanium. The reasons for these procedures are to achieve methodological advantages when evaluating the responses of biomolecules, cells and tissues to titanium. By coating a glass surface with metallic titanium an extremely flat surface suitable for spectroscopy may be obtained. By coating a polymeric material like epoxy resin or polycarbonate you allow for sectioning of the coated implant material in situ together with the adjacent tissue as the solid metal is only sectional through expensive and time consuming grinding techniques that also restrict the subsequent microscopic analysis.
Devices have also been coated with metallic titanium to benefit from the perceived but not defined good biocompatibility of titanium. An example of many such applications is described in PCT/SE93/00924.
Current manufacturing technology can provide a wide range of materials with various physical properties but most cannot be utilized as biomaterials because of issues of biocompatibility. The response of cells of foreign materials placed within the body can lead to inflammation and rejection unless the implanted device is made of a relatively small number of materials which include titanium and Ti-6Al-4V alloy. This small selection of biocompatible materials limits the design and development of devices which can be implanted in the body.
Since titanium is a metal it does not have the wide variation of physical characteristics which polymers can achieve. Yield strength, elastic modulus and elongation are some of the factors which can be varied more easily in polymeric materials compared to metallic materials.
Also, titanium is not suitable in biosensors. They have to be constructed with other materials than titanium to achieve their function. Many such sensors must have semi permeable membranes to allow the exchange of molecules to be monitored.